1. Field of the Invention
The present invention relates generally to methods of preventing gas migration during primary well cementing, i.e., the cementing of a string of pipe in a well bore.
2. Description of the Prior Art
After a well bore is drilled by rotary drilling wherein a drilling fluid is circulated through the well bore, the circulation of the drilling fluid is stopped and a string of pipe, e.g., casing, is run in the well bore. After the pipe is run, the drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus between the exterior of the pipe and the walls of the well bore while removing drilling solids and gas therefrom. The next operation performed on the well is usually primary cementing. That is, the string of pipe disposed in the well bore is cemented therein by placing a cement slurry in the annulus between the pipe and the walls of the well bore. The cement slurry is permitted to set into a hard substantially impermeable mass in the annulus whereby the pipe is bonded to the walls of the well bore and the annulus is sealed.
Gas migration, also called annular gas flow, is the migration of gas in the annulus from one or more gas formations penetrated by the well bore during and after primary cementing. Gas migration in the annulus can cause severe problems including communication between producing zones, e.g., high volume loss of gas from a high pressure zone to a low pressure zone, and the failure of the cement to seal the annulus which in the worst case can result in an uncontrollable blow-out.
It is known that gas migration is caused by the behavior of the cement slurry during the transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. The transition phase starts when the cement slurry develops enough static gel strength to restrict the transmission of hydrostatic pressure over its column height and ends when the cement slurry develops a gel strength which is sufficient by itself to prevent migration of gas up through the cement slurry. If the pressure exerted on one or more gas formations by the cement slurry falls below the pressure of the gas in the formations, the gas enters the well bore and can migrate through the annulus. The initial gas migration causes flow channels to be formed in the cement slurry whereby the migration continues after the cement slurry sets.
Gas migration can be prevented during primary cementing if the pressure exerted by the cement slurry on gas formations penetrated by the well bore is maintained at a level greater than or equal to the pressure in the gas formations during the transition phase of the cement slurry.
Prior attempts to prevent gas migration have included the use or higher density drilling fluids and cement slurries, applying pressure to the annulus at the surface, utilizing more effective fluid loss control additives to prevent cement slurry volume reduction, the generation of a volume of high pressure gas within the cement slurry to increase its compressibility, entraining gas in the cement slurry at the surface as it is introduced into the well bore and the like. While all of the these techniques have proved to be beneficial, none have completely solved primary cementing gas migration problems. Thus, there continues to be a need for improved methods of preventing gas migration during primary well cementing.